--> Western Interior Seaway (WIS) — Upper Turonian-Lower Coniacian Chronostratigraphy and Sequence Stratigraphy: Significance of E/T Disconformities, Relative Sea-Level Oscillations and Eustasy in the Cardium and Niobrara Formations

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Western Interior Seaway (WIS) — Upper Turonian-Lower Coniacian Chronostratigraphy and Sequence Stratigraphy: Significance of E/T Disconformities, Relative Sea-Level Oscillations and Eustasy in the Cardium and Niobrara Formations

Abstract

New Upper Turonian and Lower Coniacian molluscan fossils collections from Alberta and Colorado provide the biozonal building blocks with which we assemble the first regional chronostratigraphic (Wheeler-Grabau) diagram for the Niobrara and Cardium formations. These Coniacian substages encompass 2.7 Ma (U. Turonian 1.6 Ma & L. Coniacian 1.1 Ma). Key to reconstruction is the bivalve biozone of the Late Turonian subage, the Cremnoceramus walterdorfensis walterdorfensis biozone, which lasted 100 ka. Additional precision is provided by the Lower Coniacian substage that is subdivided with 5 bivalve biozones instead of 1 ammonite biozone. With this data the allostratigraphic chronology for 10 Cardium Formation “E” disconformities and their regional consequences can be validated. Notably Cardium Formation disconformities “E5.5-7” have been correlated to erosional surfaces in the Niobrara Formation, with intracontinental “E” disconformities extending along 1650 km of the WIS. Importantly while “E” disconformities are thought to be eustatically controlled relative sea-level oscillations, our chronostratigraphic placement establishes periods when the WIS would have been reorganized by these events. In this context, disconformity “E5.5” has the largest time gap (lacuna). At Horseshoe Dam, Alberta, the “E5.5” lacuna ranges > 500 ka. In contrast, overlying disconformities “E6-7” at Horseshoe Dam appear to range < 50 ka each, but at Pueblo, Colorado, in the center of the WIS, they amalgamate and their combined lacunal range is > 200 ka. Thus the WIS may have been exposed for ≥ 100 ka during the Early Coniacian and > 250 ka during the latest Late Turonian. Under this interpretation important disconformity paleosol horizons should be present, but none have been reported to date. Other researchers attribute this absence to efficient removal of paleosols during transgressions. The impact of eustatic control can also be tested if eustasy was driven glacially. For example, during the eustatically controlled Pleistocene glaciation, sea level fell 120–130m, an event where ∼45 Mkm3 or 3% of global oceanic water was transferred into ice masses and continental lakes. In contrast, if the WIS had been 300 m deep during the Late Turonian and Early Coniacian then 108 Mkm3 or 8% of oceanic water would have been transferred into ice caps and continental lakes. This large glacial ice volume should have left a significant sedimentary record in WIS, but to date none has been reported.